Sulfidation resistant cobalt-base alloys



United States Patent 3,549 354 SULFIDATION RESISTANT COBALT-BASE ALLOYS Claude R. Wheeler, Phoenix, Ariz., assignor to The Garrett Corporation, a corporation of California No Drawing. Filed May 29, 1968, Ser. No. 732,898 Int. Cl. C22c 19/00 US. Cl. 75-1225 9 Claims ABSTRACT OF THE DISCLOSURE A sulfidation resistant cobalt-base alloy comprises a major amount of cobalt, between about and about 30% chromium, between about 0.05% and about 3.0% by weight of an actinide series metal, between about 1.0% and about 6.0% by weight aluminum and preferably also containing tungsten and/or molybdenum, columbium and/ or tantalum and carbon.

BACKGROUND OF THE INVENTION There has been a search for some time for metal alloy compositions which are resistant to the high-temperature corrosive combustion products of gas turbines. Such deterioration referred to hereinafter as sulfidation takes place particularly in the heated sections of gas turbines which are exposed to gaseous combustion products of sulfur containing fuels such as fuel oil, diesel fuel, aviation gasoline and the like. The metallic parts on which deterioration by sulfidation is greatest consists of machined parts, such as gas turbine wheels and/or blades, and nozzle vanes, which are directly exposed at high temperatures to the sulfur containing gaseous combustion products. Sulfidation corrodes these parts especially after long exposures to the combustion gases at high temperatures thereby not only weakening the metallic structures, but literally decomposing substantial portions, Such corrosion not only results in continued deterioration of the turbine engine efliciency but in time may result in actual failure of the engine to produce sufficient thrust to be useful. This problem is especially serious under desert or salt water atmosphere conditions where the presence of alkali or alkaline-earth metal salts such as sodium chloride, even in concentrations as low as 0.1 part per million, are found to catalyze the sulfidation reaction.

Although certain nickel-base alloys have been known to provide good oxidation resistance under high-temperature conditions, known cobalt-base alloys are rather poor by comparison. Addition of aluminum to a chromium containing cobalt-base alloy has been found to improve oxidation resistance. However such compositions still do not possess satisfactory sulfidation resistance at high temperatures. On the other hand, the presence of a small amount of thorium actually provides practically no improvement in oxidation and sulfidation resistance to the cobalt-base alloys in the absence of aluminum.

The present invention relates to the discovery that thorium or other actinide series metal in combination with chromium and aluminum results in extremely high sulfidation and oxidation resistant cobalt-base alloys. This fact is further surprising since nickel-base alloys which have nominally the same composition as the sulfidation resistant cobalt-base alloys of the invention, with the replacement of cobalt with nickel, possess sulfidation resistance which is sometimes worse than the nickel-base alloy containing no thorium.

DESCRIPTION OF THE INVENTION The superior sulfidation resistant alloys of the invention are cobalt-base alloys containing a small amount of actinide series metal, chromium and aluminum and preferably other additional metals which enhance the strength properties of the alloy. By the term cobalt-base alloy as used herein and understood in the art, is meant a metallic composition wherein the major metal present is cobalt and preferably wherein cobalt is present in a major amount. More specifically, it has been found, according to the invention, that cobalt-base alloys containing from about 0.05 to about 3.0% by weight actinide series metal, at least about 5.0% chromium and from about 1.0 to about 6.0% by weight aluminum possess outstanding sulfidation as well as oxidation resistance.

The metal used in small quantities as a synergistic sulfidation inhibitor in combination with chromium and aluminum for cobalt-base alloys according to the invention consists of an actinide series metal. The actinide series metals within the scope of the invention in addition to actinium, are thorium, protactinium and uranium. Other metals included in the actinide series are not regarded as within the scope of the invention in view of their high radioactivity and non-availability. Thus, the synergistic metals will hereinafter be referred to as naturally occurring actinide series metals. Thorium and depleted uranium in which radioactive isotopes have been removed are preferred. Thorium is particularly preferred due to its availability, relatively high melting point and high sulfidation resistant properties in combination with aluminum. Concentration of these metals between about 0.05 and about 3.0% is critical. The presence of less than about 0.05% is insufficient to obtain substantial increase in the sulfidation resistance of the alloys at elevated temperatures. However, concentrations above of about 3.0% by weight of the metallic alloy are believed to result in grain boundary segregation which causes embrittlement of the metal during hot working, hot shortness in casting and general deterioration of mechanical properties. The metals may be used in their essentially pure form or on the other hand, may be present in mixtures.

The concentrations of aluminum and chromium are also somewhat critical. Amounts of aluminum below about 1% do not offer sufficient corrosion resistance whereas concentrations above about 6% are found to result in brittle alloys of questionable use. The presence of chromium in amounts of at least about 5% by weight is necessary to achieve desired sulfidation resistance. Concentrations between about 5 and about 30% are preferred with amounts between about 10 and about 25% offering both superior sulfidation resistant and high strength alloys.

Preferably, the cobalt-base alloys of the invention contain between about 50 and about cobalt. In order to provide high strength cobalt-base alloys it is also necessary for the compositions to contain tungsten and/or tantalium wherein the total amount of this group of metals is between about 3 and about 20% by weight of the alloy. The preferred amounts of these metals individually are between 0 and 15% and more preferably between about 4 and about 10% individually for tungsten and tantalum with the total amount of this group of metals being no more than about 20% by weight of the alloy composition. Alternatively, molybdenum may be present to replace all or part of the tungsten and columbium may be present to replace a part or all of the tantalum. Molybdenum, preferably, may be present up to about 10% and columbium up to about 2% by weight of the alloy. The presence of a small amount of carbon also increases the strength of the alloy. However, where the carbon content is above about 0.5% by weight, oxidation resistance is sacrificed somewhat. Suitable carbon concentrations are between about 0.05 and about 0.5% by weight.

A convenient manner of expressing the concentrations of the preferred cobalt-base alloys of the invention is by mole ratios based on cobalt. Thus, the preferred alloy compositions corresponding to the concentrations set forth above are as follows:

Metal: MetalzCo Al+X 1:50-127 Cr 1 71:2.5 W, Ta, Mo, Cb l:61:3

where X represents an actinide series metal.

The following examples are provided to illustrate the manner in which the invention is carried out. Unless otherwise specified, parts and percents set forth are given by weight.

EXAMPLE I 4 EXAMPLE 11 Additional alloys were prepared and tested for sulfidation resistance at 1950 F. according to the procedure of Example I.

Alloy composition Weight loss,

Co W C Cb+Ta Cr Al Th mg./sq. em.

The results show the criticality and marked superiority of the alloys of the invention.

EXAMPLE III Samples of alloys prepared and tested according to Example I were subjected to high-temperature oxidation at 2000 F. for 115 hours. The oxidation atmosphere consisted of slowly moving air. The samples were then cooled, cleaned with caustic, wire brushed and weighed to determine the weight change caused by the oxidation. The oxidation test results are shown in Table II.

TABLE II specimens were placed in a silica retort which was in turn Weight change, placed in the furnace containing the aforementioned gases L 9%. for two hours. For the following 20 hours the furnace was 1000 2100 supplied with a mixture of 2.0 c.f.h. air, 0.072 c.f.h. C0 3 and 0.013 c.f.h. so which gases were bubbled through a 3 10% aqueous sodium chloride solution. Accordingly, the 2% -2 gaseous atmosphere within the furnace contained about 1 p.p.m. sodium chloride. During the 22 hours of the test the temperature of the furnace was maintained at an ele- EXAMPLE Iv vated temperature as set forth below. Following the ex- By way of comparison, the sulfidation test of Example I posure to the high temperature conditions the alloy at 1950 F. was repeated utilizinga typical high-oxidation were cooled and cleaned and weighed to determine the resisting nickel-bas alloy k wn a InC n 1713C having weight loss caused by sulfidation corrosion. The use of the following analysis: Percent tWO different atmospheres during the test duplicates a gas Ni 42 turbine engine combustion exhaust whereupon at start up 125 XC S H 5 is present and thereafter during continuous Mo 42 Operation S0 is present in the gaseous exhaust mixture. Cb Cobalt-base alloys were prepared and subjected to the Ti 8 severe sulfidation test set forth above. The master melt 1 6 1 alloys A, E and G were modified by the addition of 4? B 0.012 aluminum and/or thorium. The alloys had the following Zr 0.010 analysis: C 0.12

Co Cr W Zr Ta-l-Cb C Ni Fe Mn Al Th Sulfidation corrosion for the alloys at the test temperatures indicated was as follows:

Weight loss, rug/sq. cm.

1,800 F. 1,950 F. 2000 F.

Similar results are obtained by substituting depleted uranium for thorium.

The alloy was further modified by the addition of 0.1% thorium and tested for sulfidation resistance at 1950 F. according to the procedure of Example I.

Inconel 713C: Weight loss-mg./sq. cm., 295. Inconel 713C F0.1% Th: Weight 1ossmg./sq. cm., 299. The results show that nickel-base alloys containing chromium, aluminum and thorium do not possess sulfidation resistance as compared to the cobalt-base alloys of the invention.

The alloys of the invention may also contain small amounts of phosphorus and sulfur and preferably in amounts not over about 0.04%, siilcon in an amount not over about 0.5% by weight. Nickel may be present in small amounts up to about 10% as Well as iron and manganese in amounts up to about 2.5% in addition to small amounts of zirconium and boron. However the absence or presence of these additional elements is not critical or necessary to the alloys of the invention.

The superior sulfidation resistant cobalt-base alloys as disclosed herein may be used for a number of varied purposes. The alloy may be casted, rolled into sheets or prepared as bars or tubing for any uses where superior resistance to sulfur attack as well as oxidation is desired.

What is claimed is:

1. A cobalt-base alloy comprising at least about by weight chromium, a small effective amount of aluminum and a naturally occurring actinide series metal in combination, the amount of chromium, aluminum and actinide series metal being effective to increase the sul fidation resistance of the cobalt-base alloy and the remaining balance consisting essentially of a majority of cobalt.

2. The composition of claim 1 wherein the amount of aluminum is up to about 6.0% by weight and the amount of said actinide series metal is up to about 3.0% by weight.

3. The composition of claim 1 wherein the amount of aluminum is at least about 1.0% by Weight and the amount of said actinide series metal is at least about 0.05% by weight. 4. The composition of claim 1 which contains one or more metals selected from the group consisting of tungsten, tantalum, molybdenum and columbium wherein the total amount of the group is between about 3% and about 20% by weight of the total composition.

5. The composition of claim 4 wherein the respective individual amounts of tungsten and tantalum are between about 4 and 10%, the amount of molybdenum is up to about 10% and the amount of columbium is up to about 2% by weight and the total amount of the group remains between about 3% and about 20% by weight of the total composition.

6. The composition of claim 1 which contains up to about by weight chromium.

7. The composition of claim 1 which contains up to about 0.5% by weight carbon.

8. The composition of claim 1 wherein the actinide series metal is selected from the group consisting of depleted uranium and thorium.

9. The composition of claim 1 wherein the actinide series metal is thorium.

References Cited UNITED STATES PATENTS 8/1968 Roush 171 1/ 1969 Kawahata et a1. 75-17l US. Cl. X.R. 

